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Alternator test lead

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bbradburry(at)allvantage.
Guest

Posted: Mon Oct 02, 2006 4:46 am Post subject: Alternator test lead

A couple of days ago, Bob referred to a method to attach a test lead to

the alternator field wire to do easy troubleshooting of the alternator.

A 1K ohm, 1/2 watt resistor was specified to isolate the test lead from

the field.  Would this same resistor work for a 28V system, or would a

different resistor be required?



Thanks,

Bill Bradburry

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nuckollsr(at)cox.net
Guest

Posted: Mon Oct 02, 2006 6:15 am Post subject: Alternator test lead

At 08:47 AM 10/2/2006 -0400, you wrote:



 	  Quote: 	
		  

<bbradburry(at)allvantage.com>



A couple of days ago, Bob referred to a method to attach a test lead to

the alternator field wire to do easy troubleshooting of the alternator.

A 1K ohm, 1/2 watt resistor was specified to isolate the test lead from

the field.  Would this same resistor work for a 28V system, or would a

different resistor be required?


	


   ANY 1/2 watt resistor of 200 to 2000 ohms would do at any voltage.

   It's a protective measure to prevent a shorted test line from taking

   the alternator system down. You could put an in-line 1A fuse there

   too. The resistor is smaller and less 'lumpy' when inserted under

   heatshrink in the test line.



   With the resistor (or fuse) in place, a short downstream on

   the test wire won't upset the alternator system. Adding the

   resistor has a  negligible effect on readings needed to diagnose

   the system.



    Bob  . . .



      ---------------------------------------------------------

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

      ---------------------------------------------------------

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gregcampbellusa(at)gmail.
Guest

Posted: Tue Oct 03, 2006 6:16 am Post subject: Alternator test lead

This has already been answered, but to add a little perspective and some math to the answer...

  

 1) What is the purpose of the resistor in the voltage sense line?  To prevent bad things from happening if the other end of the voltage sense line (or some point in the middle) accidentally shorts to ground, or in the case of a alternator field, shorts to positive. 

  

 2) What's the "worst case" scenario for the particular application?  If you had a voltage sense lead running to say a battery, and the other end shorts to ground - it would try and draw hundreds of amps through a tiny sense wire.  If the sense wire is small, it might act like a fusible link and go poof.  (Good!)  If it is big wire (you used some spare #10 wire for "sturdiness"), then it might act like a dandy heater and fire starter.  (Bad!) 

  

 3) What's the "cure" ?  Add a resistor in line with the voltage test point that's suitable for the "worst case".  So even if you used some spare #8 wire for your voltage sense wire, but - at the battery end - you added a 1000 ohm resistor, then in a 14v system (with the alternator ON), a short would produce a maximum of  0.014 Amps or 0.196 Watts.  Not much of a heater or a fire starter. (Good!)   And at 0.196 Watts, a quarter Watt 1000 ohm resistor could happily dissipate that heat all day long without getting too hot.

  

 4) What if it's a 28v system?  At 28volts, a 1000 ohm resistor will draw 0.028Amps, and since Watts = Volts * Amps, or Amps^2 * Resistance, a paltry 0.784 Watts.  A quarter watt resistor will get extremely hot and may even fail open.  A larger 1/2 Watt resistor will get nasty hot.  A 1 Watt resistor will get toasty warm, but can handle that load all day long depending on how it's mounted.  (The Watt rating is almost always about surface area and ventilation.) 

  

 Arguably, you might prefer the resistor to act as a fusible link in a situation like that.  So a 1/8 Watt resistor might be the resistor of choice.  One good overload and "poof", no more resistor.  Of course, no more voltage sensing either - leading to erroneous conclusions as to the nature of the problem. 

  

 The best way to tell what your resistor will behave like is to short it out and see how hot it gets.  If it's going to be bundled with other wires, covered with a boot, etc..  it's best to simulate those conditions as well. 

  

 5) In the case of a voltage sense wire on an alternator field, you also have to be concerned about what happens if the voltage sense wire shorts out to positive.  If the sense wire were capable of supplying several amps to the alternator field, you could get a runaway alternator over voltage.  By adding the resistor, you prevent it from supplying more than a few tenths of an amp to the field. 

  

 6) Doing the math backwards on a 28volt system:

 a one Watt resistor of 784 ohms or higher will be fine all day long

 a 1/2 Watt resistor of 1,568 ohms or higher will be fine all day long

 a 1/4 Watt resistor of 3,136 ohms or higher will be fine all day long...

  

 For a 14 volts system, divide all the Ohms by 4...

 a one Watt resistor of 196 ohms will handle 1 Watt all day long (with proper ventilation)

 a 1/2 Watt resistor of  392 ohms will handle 1/2 Watt all day long

 a 1/4 Watt resistor of 784 ohms will handle 1/4 Watt all day long

  

  

 7) Don't fall for the common mistake of trying to "double up" resistors in parallel to get the "required" 1 Watt rating.  If you take two 1000 Ohm 1/2 Watt resistors (more readily available than 1 Watt resistors, plus they come two to the pack.)   and put them in parallel you won't be happy with the results because... 

  

 You've created a 500 ohm "resistor network"   (use your ohmmeter and you'll see)...  So on a 28v system it will draw MORE amps and MORE Watts, and each of your 1/2 resistors will be trying to dissipate 0.784  Watts and not very happy about it.

  

 So...  if you're rooting through the parts bins and all you can find are 1/4 or 1/2 Watt resistors, then use the larger ohm values.  Double the resistance value if necessary.  It won't make a difference to your voltmeter, and if you accidentally short things out - it won't create smoke & funny smells where you don't want it to. 

  

 If all you can find are two 1000 Ohm 1/4 Watt Resistors, I'd be more inclined to put them in series for an effective resistance of 2000 ohms.  This would dissipate 0.098 Watts in a 14v system, and 0.392 Watts (total) in a 28v system.  The heat dissipated by each resistor would be half that, well within the limits of a 1/4 Watt resistor. 

  

 Bob's probably right - any resistor in the 200 to 2000 ohm range will do, especially in the 1 Watt size.

 I prefer to use resistors in the 1000 to 3000 ohm range in the more common 1/4 or 1/2 Watt sizes.

 If you have a 28v System, I'd use the higher resistance values, but you can do the math.

  

 The really important thing is that you put the resistor close to the "source", 

 not at the measuring end of the "sense wire".

 It's kind of like a fuse - you want it to protect the entire run of the wire, not the "end device" 

 which is your voltmeter.

  

 Greg

  

  [quote][b]

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